High quality threads are commonly cut in a workpiece mounted on a lathe, by a threading insert that is held in a tool holder. The cutting edge of the insert has a limited life, such as 50 threads per cutting edge, and the life of the insert is increased by constructing it with multiple cutting edges. When a cutting edge wears out, the insert is removed and reinstalled with a new cutting edge in position, it being highly desirable that the new cutting edge lie precisely in the same position as the old edge, despite the change in insert position. The manufacturing cost for an insert is about the same no matter how many cutting edges it has, so it is desirable that each insert have as many cutting edges as possible. However, any insert, to be practical, has to allow rapid and highly precise reinstallation of the insert to present a new cutting edge, with the insert and the tool holder being of simple and rugged design. It is also desirable if the insert can be used to cut an internal thread in a hole of small diameter, as well as an external thread.
Stier U.S Pat. No. 3,613,197, describes a threading insert with eight cutting edges. His insert includes a mount portion with a screw-receiving hole for mounting on a tool holder, and four arms radiating from the mount portion, with each arm having two cutting edges. However, he precisely locates his insert on a tool holder by using locating surfaces along the opposite sides of each arm. As a result, the sides of his arms are long, resulting in only a small thickness of metal between the innermost locations on his arms and the central hole of the insert, Also, the chips generated during cutting ride along a long flat surface into a narrow area, resulting in "balling" up of the chips. A threading insert which provided many cutting edges, which was of sturdy construction and could be precisely mounted on a tool holder of simple construction, would be of considerable value.